This invention relates to acousto-optic polarization converters, and more particularly, to the use of acousto-optic polarization converters in a multi-channel wavelength-routing switch.
Acousto-optic polarization converters are well known in the art. In such prior art converters, a transducer of interdigitated electrodes is formed at the surface of a piezo-electrical material and is electrically driven by an RF-frequency signal to launch an acoustic wave at the surface of the material. For an appropriate crystal orientation and direction of propagation, the surface acoustic wave provides for quasi-phase-matched conversion between orthogonally polarized eigenstates having substantially different refractive indexes. The interaction rotates the polarization of that wavelength of light for which the momentum mismatch between polarization states nearly exactly matches the acoustic wave momentum. Placing the converter between crossed broad-band polarizers allows it to operate as a narrow-band acousto-optic tunable filter (AOTF). Alternatively, a device consisting of an input and output polarization beam splitter and two similarly driven acousto-optic polarization converters functions as a switch, which directs polarization transformed wavelengths to one output port and untransformed wavelengths to a second output port.
Much effort has been directed in the prior art to improving the passband transmission characteristics of acousto-optic polarization converters. One major concern of the prior art has been reducing the magnitude of the side lobes in the passband, which deleteriously affect the performance of the converter as either a filter or as a switch. Various apodized acousto-optic converters are disclosed in the prior art which redistribute the acoustic energy interaction with the optical signal in such a manner that the undesirable side lobes am reduced. For example, in U.S. Pat. No. 5,218,653 to Johnson et al., incorporated herein by reference, a polarization converter is disclosed with an apodized acoustic waveguide in which a surface acoustic wave is launched in one surface acoustic waveguide which is directionally coupled to a second surface acoustic waveguide in the middle of which runs an optical waveguide. The interaction length in the second acoustic waveguide is such that the power density of its acoustic wave spatially varies from a minimum to a maximum and back to a minimum. This length is one-half the coupling length, L.sub.c, of the waveguide, which is a function of the characteristics of the acoustic waveguides. Thereby, the acoustic energy in the second acoustic waveguide is apodized and the side lobes of the interaction with the optical signal are reduced.
Also of concern is the "flatness" of the passband. The most desirable passband has unity transmission over the wavelength to be passed, and zero transmission elsewhere; the corresponding rejection band is both deep and broad, i.e,. essentially no light is transmitted into unselected channels. Such a passband can accommodate the inevitable variations in signal wavelength, and will not narrow when several filters are used in series. The shape and depth of the rejection band is critical when the converter is used as a switch where a small increase in cross-state loss results in a large increase in bar-state crosstalk.
Co-pending patent application Ser. No. 08/264,674 filed Jun. 23, 1994, which is incorporated herein by reference, discloses a passband-flattened apodized acousto-optic polarization converter that is based on an extension and modification of the afore-noted polarization converter disclosed in U.S. Pat. No. 5,218,653. In this converter, passband flattening is achieved by introducing a region in which the sinusoidally varying acoustic amplitude is both phase-reversed and diminished. This is accomplished by placing a partial acoustic absorber rather than a complete absorber at a crossover length, L.sub.x, equal to one-half the coupling length, and increasing the interaction length of the converter to 2L.sub.x.
As noted, the shape, depth and location of the rejection band is of particular concern when acousto-optic polarization converters are used to switch wavelengths. This is best explained with reference to FIG. 1, which shows a plan view of a prior art optical switch 101 incorporating two essentially identical and similarly driven apodized acousto-optic polarization converters 102 and 103, of the type described in afore-noted U.S. Pat. No. 5,218,653. An input optical signal of any polarization is inputed on fiber 104 to a polarizing beamsplitter 105, which splits the input signal into two orthogonally polarized output signals on polarizing maintaining fibers 106 and 107. The orthogonally polarized signals on these two fibers are then inputed to polarization converters 102 and 103, respectively. Depending upon the wavelength of the optical input signal and the wavelength of the surface acoustic wave applied to the first surface acoustic waveguides 114 and 115 in each converter, each polarization converter either converts or doesn't convert the polarization of its input signal to the opposite polarization. If the polarizations are both not convened by the converters 102 and 103, then the light in each of the polarizing maintaining fibers 108 and 109 at the outputs of converters 102 and 103, respectively, has the same polarization as in its corresponding input fiber. Output polarizing beamsplitter 110 then combines these oppositely polarized signals on fibers 108 and 109 to produce a signal at the first switch output on output fiber 111. If, on the other hand, converters 102 and 103 convert polarizations, then the light in each of the polarizing maintaining fibers 108 and 109 is opposite in polarization to that in its corresponding input fiber and beamsplitter 110 combines these oppositely polarized signals to produce a signal on the second switch output on output fiber 112. Thus, depending on its wavelength, an input signal is either not switched and is outputted on fiber 111, or is switched and is outputted on fiber 112.
FIGS. 2A and 2B shows the unswitched (bar-state) and switched (cross-state) transmission versus wavelength characteristics of switch 101. As can be noted in FIG. 2A, all wavelengths are passed to the unswitched output except for wavelengths within the narrow band surrounding the wavelength .lambda..sub.1 at which polarization conversion takes place. As noted, this optical wavelength, .lambda..sub.1, is determined by a corresponding acoustic wave of wavelength, .LAMBDA..sub.1, imposed on the first acoustic waveguides 114 and 115 in converters 102 and 103, respectively. At .lambda..sub.1, transmission is shown to be 20 dB below the 100% transmission at other wavelengths. Similarly, the switched output in FIG. 2B shows transmission at 100% only at a narrow band surrounding .lambda..sub.1, and falls rapidly outside the band. An input channel on input fiber 104 at wavelength .lambda..sub.1 thus has most of its optical energy switched to the second switch output on fiber 112, while simultaneous optical channels at other wavelengths remain unswitched and pass to the first switch output on fiber 111.
In an optical network, in addition to the multiple wavelength channels inputed to switch 101 on fiber 104, additional channels at the same or different wavelengths are likely to also be inputed to a second input of polarizing beamsplitter 105 on a second input fiber 113. These channels at other than the switching wavelength .lambda..sub.1 pass unswitched to the second switch output on fiber 112, while a channel at .lambda..sub.1 is switched to the first switch output on fiber 111. Switch 101 thus acts to pass all channels on its first and second inputs to its first and second outputs, respectively, while switching channels at .lambda..sub.1 to opposite outputs.
When switching efficiency at a particular wavelength is high at approximately 99% (i.e., only 1% of the light from an input channel goes into an output channel that shouldn't have any signal at that particular wavelength), crosstalk between channels at the same wavelength will be low. If switching efficiency should decrease, to 90% for example, then 10% of the light is improperly directed; this is a significant increase in improperly directly light that will interfere with a proper signal at that wavelength and deleteriously affect crosstalk performance. Thus the depth of the bar-state (FIG. 2A) is critical in switch performance. Also critical to switch performance is the width of the rejection band which must be across the entire wavelength defined channel in order to accommodate expected channel variations.
Both rejection depth and width and/or placement of the rejection band become critical factors when the acousto-optic polarization converter is used as a multichannel wavelength-routing switch. In 5particular, when switch 101 is used to simultaneously switch several closely-spaced wavelengths, all lying withing a wavelength channel, by imposing multiple acoustic waves of different wavelengths on the acoustic waveguides 114 and 115, switching efficiency has been found to decrease. Specifically, it has been experimentally determined that when neighboring channels which are close in wavelength are simultaneously selected for switching, the depth of rejection of their bar-state transmissions decreases. Furthermore, the channels are shifted toward each other by a small, but potentially harmful amount. FIG. 3 shows the measured bar-state transmission of an acousto-optic converter when two channels separated by 4 nm are switched together and when each channel is separately switched. The reduction in the depth of rejection of each channel when simultaneously switched and the shift of channel positions toward each other can be noted.
It was experimentally noted that closely spaced channels always move towards each other. In addition, it was also experimentally noted that by shifting the wavelength of the second channel far away from the wavelength of the first channel, the first channel returns to its unperturbed position. These factors led to the conclusion that the cause of the shifts is not thermal, which would be independent of channel spacing and would move all channels in the same direction. Rather, the channel shift can quantitatively be attributed to the fact that the response of the polarization to the acoustic amplitude is not linear (i.e., a change in acoustic amplitude does not produce a proportional change in the polarization conversion, and indeed, in some cases an increase in acoustic amplitude can produce a decrease in the target polarization).
The consequences of these shifts and of the reduced rejection depth can be particularly severe in networks in which a wavelength channel (rather than a single set of wavelengths) is defined. Any realistic application requires that the switch must maintain some minimum performance (e.g. worst-case crosstalk) over a finite bandwidth. In a switch having a response such as shown in FIG. 3, crosstalk, and changes in crosstalk as the channel shifts both strongly depend upon where in that channel any given wavelength lies. In cases where two separate signals, both having wavelengths lying in the same channel, are expected to cross paths in a switch, the resulting increase in crosstalk cannot be eliminated using techniques such as dilation, and can make the system unworkable.
An object of the present invention is provide closely spaced multi-channel wavelength-routing using acousto-optic polarization converters.